For an external power supply, the output cable wire resistance will cause load regulation issues, as the current drawn from the load varies. Cable wire gauge and length will also affect the output voltage accuracy as the resistance of the load changes. Traditionally, in order to maintain tight output accuracy, the output cable and circuits have incorporated two remote sense wires which are able to compensate for the total cable voltage drop. Another technique used is lowering the gage (increasing the diameter) of the output wire to lessen the effects of the voltage drop. Lack of voltage drop compensation means that output cables have to be sized with large AWG wire to minimize the IR drops and that open circuit (no load) output voltage had to be programmed high to minimize the effects of the drops. Also, output cables had to be limited to a maximum length for a given wire gauge so the drops at high current did not become excessive.
Solutions using an interchangeable tip connector can accommodate for cable voltage drops, but require two remote sense wires in the cable, leading to two additional connector pins in the tip, to achieve the voltage correction. Adding a second remote sense wire and two more tip connector pins increase the weight and size of the cable and tip connector, thereby adding unneeded bulk and weight for a user. A single wire solution would have only compensated for half the drop passively and would have required an active stage to compensate for the entire drop on the positive and negative leads. The active compensation would have placed a length restriction on an output cable as well.
Voltage programming, as done in Mobility Electronics'Juice 70 product line, uses a resistor in a tip which is connected in parallel with a voltage sampling potentiometer chain in a base unit. The tip resistor shares a common ground with the high current ground return, so any load transients in the cable ground lead are transmitted back to the base unit and affect the output voltage. The voltage programming was limited to a pre-set minimum voltage defined by the reference in the base unit. This meant that no passive tip could force the base unit output voltage below the preset minimum (thus, no ability to upgrade the power supply to operate over a wider voltage range other than changing the base unit). The preset minimum on the Juice products is 15 volts. The voltage programming resistor in the tip was included in the overall feedback loop along with the cabling from the base unit to the programming resistor. This puts a portion of the feedback loop outside the base unit, making loop compensation more difficult.
FIG. 1 discloses a 4-wire prior art example of a voltage programming system 100. In this example, note that load current, IL, flows through the ground path shared by R122. Noise or transients on this ground or picked up by the Vsense line cannot be filtered by the addition of bypass capacitors (across R114 or R122) as this would place an extra pole on integrator U1 outside its local feedback loop. This external low frequency pole would make loop compensation difficult and degrade the transient response of the base unit. Also, the distributed capacitance of the cable itself produces a similar pole at higher frequencies, again contributing to problems in compensating the feed back loop.